This invention relates generally to devices for use in crystal growth by the Czochralski method, and more particularly, to an apparatus for pulling the crystal from semiconductor source melt.
In operation of the Czochralski method of single crystal growth, a quantity of source material is formed in a crucible by melting solid semiconductor source material maintained in an inert environment. A crystal is attached to a cable and lowered into the melted source material. As the crystal grows on the seed it is slowly pulled from the melt.
The cable is typically wrapped around a drum which is rotated to slowly pull the crystal from the melt as it grows. As the weight of the crystal increases the tension on the cable increases. If the rate at which the crystal weight and cable tension increases is greater than the rate at which reactive friction forces between the cable and drum are generated, the cable will momentarily slip on the drum until the tension load created in the cable wrapped on the drum combined with the friction force is at least equal to the increase in the weight of the crystal and equilibrium is established. When this slip occurs, the crystal drops into the source material melt and creates a splash wave on the melt surface. The lowering of the crystal back into the source material melt and the wave on the melt surface may cause a loss of zero dislocation structure in the crystal. After a first dislocation is formed in the crystal it can multiply, generating numerous dislocations which spread out into the crystal. Therefore, if a dislocation-free growing crystal is disturbed at one point by slipping of the cable on the drum, the whole cross section and a considerable part of the already grown crystal may be inundated with dislocations.